Yeah, space has a coolness factor, and there are things about the origin of the universe we can't learn from the bottom of the ocean, but an underwater station is one of the most underrated projects humanity could pursue.
Oceans are phenomenal shields against radiation, extreme weather, or even the temporary combustion of the atmosphere, should things get that bad. The opportunity to learn about groups surviving in isolated habitats, or about megastructures that have to survive hostile external environments, could even provide really useful lessons for space.
If we ever find ourselves in a situation where we cannot survive on a planet where food literally grows out of the ground, with abundant liquid water and atmospheric oxygen at 1 atm, how on Earth would we survive on a planet that is devoid of life, has no liquid water, an atmosphere that's 1% the pressure of Earth's with no atmospheric oxygen, and little to no sources of energy?
Again, the bottom of the ocean is far more habitable than Mars, even if we experienced a similar extinction event similar to the one we experienced 65 million years ago.
Earth is habitable now. There are plenty of plausible non-anthropogenic disasters that may make that so not so. Some typical examples include megavolcano eruption, coronal mass ejection, and asteroid impact. It would be good to not have all of our eggs in one basket here on Earth.
And it's for exactly the same reason of having redundancy that it would be a good thing for Earth to continue to remain habitable, i.e. we should stop ruining the planet through climate change.
It's not a moral hazard at all; the two outlooks are congruous.
Even after any of those events, the bottom of the ocean would still be more habitable than Mars.
> It's not a moral hazard at all; the two outlooks are congruous.
The moral hazard is that we'll begin to think of Mars as a backup, when it isn't one at all. The Earth is very likely to be the one shot we have at a home. If we can't manage to make Earth habitable in the face of catastrophe, there is little hope to make a cold, barren rock that's millions of miles away habitable.
We could create an oxygen atmosphere on Mars much more quickly than that, because we'd be sending life explicitly for that purpose that's ready now, and we'd also be importing gases and digging up in-situ resources and turning them into gas.
No, it wasn't. Photosynthesis that released O2 evolved relatively quickly, and was more efficient than other forms of photosynthesis. Then it took another billion years for oxygen to accumulate in the atmosphere. Mind you, we had an entire planet with oceans teeming with photosynthetic life. That will be next to impossible to replicate on Mars.
There are two oxygen sinks on Earth that reacted with oxygen released from photosynthesis: the first was the ocean, the other was minerals on land. This is the reason that it took so long for atmospheric oxygen to accumulate. Mars will have a lot of oxidizable minerals and compounds that will react with oxygen before it can accumulate in its atmosphere. Even then, it will escape into space without a sufficient magnetosphere to trap an atmosphere.
The bottom of the ocean would be far more habitable if we faced any type of extinction event.